1. Field of the Invention
The present invention relates to microwave-interactive packaging materials. In particular, the present invention relates to safe and abuse-tolerant microwave shielding structures in packaging materials for cooking microwavable food.
2. Description of the Related Art
Although microwave ovens have become extremely popular, they are still seen as having less than ideal cooking characteristics. For example, food cooked in a microwave oven generally does not exhibit the texture, browning, or crispness that are acquired when food is cooked in a conventional oven. In other instances, uneven cooking is exhibited wherein portions of the food may be overcooked or undercooked, soggy or dried out.
A good deal of work has been done in creating materials or utensils that permit food to be cooked in a microwave oven to obtain cooking results similar to that of conventional ovens. The most popular device used at present is susceptor material, which is an extremely thin (generally 20 to 100 xc3x85) metallized film supported on a dimensionally stable substrate that heats under the influence of a microwave field. Various plain susceptors (typically aluminum, but many variants exist) and various patterned susceptors (for example, square matrix, flower-shaped, hexagonal, slot matrix, and xe2x80x9cfusexe2x80x9d structures) are generally safe for microwave cooking. However, susceptors do not have a strong ability to modify a non-uniform microwave heating pattern in food, for example, by shielding or redistributing microwave power. The quasi-continuous electrical nature of susceptor material prevents large induced currents and thereby limits its power reflection capability, which is generally on the order of 50-55% reflection of incident microwave energy. Commonly owned U.S. Pat. No. 6,133,560 approaches the problem by creating low Q-factor resonant circuits by patterning a susceptor substrate, which provides a limited degree of power balancing. Regardless, the ability of susceptor material alone to obtain uniform cooking results in a microwave oven is limited.
Electrically xe2x80x9cthickxe2x80x9d or xe2x80x9cbulkxe2x80x9d metallic materials (e.g., foil materials) have also been used for enhancing the shielding and heating of food cooked in a microwave oven. For example, a solid foil sheet provides 100% reflection of microwave energy, thus completely shielding the food product. Foil materials are much thicker layers of metal than the thin, metallized films of susceptors. Foil materials, also often aluminum, are quite effective in the prevention of local overheating or hot spots in food cooked in a microwave by redistributing the heating effect and creating surface browning and crisping in the food cooked by the heat generated in the induced currents around the edge of the foil. However, many designs fail to meet the normal consumer safety requirements by causing fires or charring packaging, or creating arcing as a result of improper design or misuse of the material.
The reason for such safety problems is that any bulk metallic substance can carry very high induced electric currents in response to a high, applied electromagnetic field in a microwave oven cooking environment. This results in the potential for very high induced electromagnetic field strengths across any current discontinuity (e.g., across open circuit joints or between the packaging and the wall of the oven). The larger the size of the bulk metallic materials used in the package, the higher the potential induced current and induced voltage generated along the periphery of the bulk metallic substance. The applied E-field strength in a domestic microwave oven might be as high as 15 kV/m under no load or light load operation. The threat of voltage breakdown in the substrates of food packaging as well as the threat of overheating due to localized high current density may cause various safety failures. These concerns limit the commercialization of bulk foil materials in food packaging.
Commonly owned U.S. Pat. No. 6,114,679 offers a means of avoiding abuse risks with aluminum foil patterns. The structure disclosed addresses the problems associated with bulk foil materials by reducing the physical size of each metallic element in the material. Neither voltage breakdown nor current overheat will occur with this structure in most microwave ovens, even under abuse cooking conditions. Abuse cooking conditions can include any use of a material contrary to its intended purpose including cooking with cut or folded material, or cooking without the intended food load on the material. In addition, the heating effectiveness of these metallic materials is maximized through dielectric loading of the gaps between each small element that causes the foil pattern to act as a resonant loop (albeit at a lower Q-factor than the solid loop). These foil patterns were effective for surface heating. However, it was not recognized that a properly designed metallic strip pattern could also act to effectively shield microwave energy to further promote uniform cooking.
An abuse-tolerant microwave packaging material that both shields food from microwave energy to control the occurrence of localized overheating in food cooked in a microwave, and focuses microwave energy to an adjacent food surface, was disclosed in U.S. Pat. No. 6,204,492B1. To create this abuse-tolerant packaging, one or more sets of continuously repeated microwave-interactive metallic segments are disposed on a microwave-safe substrate. Each set of metallic segments defines a perimeter equal to a predetermined fraction of the effective wavelength in an operating microwave oven. Methodologies for choosing such predetermined fractional wavelengths are discussed in U.S. Pat. No. 5,910,268, which is hereby incorporated herein by reference. The metallic segments can be foil segments, or may be segments of a high optical density evaporated material deposited on the substrate. Each segment in the first set is spaced from adjacent segments so as to create a (DC) electrical discontinuity between the segments. Preferably, a set of metallic segments defines a five-lobed flower shape. The five-lobed flower shape promotes uniform distribution of microwave energy to adjacent food by distributing energy from its perimeter to its center. This abuse-tolerant packaging design on average achieves between 70-73% reflection of the incident microwave energy.
The present invention relates to an abuse-tolerant, reflective shielding pattern for use in microwave packaging materials and a method of its manufacture. The abuse-tolerant pattern is substantially opaque to incident microwave energy so as to increase reflection of microwave energy while allowing minimal microwave energy absorption. A repeated pattern or array of solid, microwave energy reflective shapes can shield microwave energy almost as effectively as a continuous bulk foil material, while resisting abuse due to cuts or tears in the packaging material or cooking without the food load. In the present invention, the abuse-tolerant array of reflective shapes achieves between 80-85% reflection of the incident microwave energy. The array of solid reflective shapes can be made of foil or high optical density evaporated materials deposited on a substrate. High optical density materials include deposited metallic films that have an optical density greater than one.
The reflective shapes prevent large induced currents from building at the edges of the material or around tears or cuts in the packaging material, thus diminishing the occurrences of arcing, charring, or fires caused by large induced currents and voltages. The reflective shapes are formed in an array, wherein each shape acts in concert with adjacent shapes to reflect a substantial percentage of the incident microwave radiation, thus shielding the food product locally and preventing overcooking. In the absence of a dielectric load (i.e., food), the microwave energy generates only a small induced current in each reflective shape and hence a very low electric field strength close to its surface, reducing the likelihood of arcing. With introduction of a dielectric food load, the current is even further reduced, enhancing the abuse tolerant properties.
Preferably, the power reflection of the abuse-tolerant reflective material is increased by combining the material in accordance with the present invention with a layer of conventional susceptor film. In this configuration, a higher surface heating environment is created through the additional excitement of the susceptor film. However, the power transmittance directly toward the food load through an abuse-tolerant reflective material according to the present invention is dramatically decreased, which leads to the shielding functionality. In the absence of food contacting the material, according to the present invention, the reflective shapes are sized such that low currents and minimal E-fields and voltage gaps are created with respect to the microwave power radiation. Thus, the chances of arcing or burning when the material is unloaded or improperly loaded are diminished.